Autologous blood transfusion systems (autotransfusion devices) are widely used for the recovery and washing of salvaged blood collected from a patient particularly at a surgical site, where a large volume blood loss can be expected—e.g. aneurysm, total joint replacement, and spinal surgeries. In such systems the salvaged blood is temporarily stored in a blood collection reservoir for re-use. A reliable determination of the liquid level or volume of the salvaged blood in the blood collection reservoir is crucial for surgeries. Inter alia it is important to avoid a complete depletion of the blood collection reservoir. Furthermore, a reliable process control in such systems requires exact information on the liquid level or volume of the salvaged blood in the blood collection reservoir.
For this purpose often the weight of the blood collection reservoir is monitored by means of a controller and upon reaching predetermined or pre-calibrated threshold values the controller outputs signals for further process control.
An example for such an approach is disclosed in U.S. Pat. No. 5,458,566, wherein the weight of the blood collection reservoir is monitored using a strain gage device to compute the volume of fluid present in the reservoir. When it is determined that a sufficient volume of fluid is present in the reservoir, the system initiates a new FILL cycle. The washing method of this autotransfusion device is not continuous but discontinuous. Thus, the response time of the volume measurement can be slower. The blood collection reservoir is supported vertically by a radially inner support member and is subject to vertical forces during system operation. The device further comprises a radially outer support member secured to a mounting surface. The strain gage is coupled between the inner support member and outer support member for sensing vertical strain forces therebetween and converting vertical forces originating with the tubes into horizontal forces. Around the blood collection reservoir many different conduits are arranged, to which personnel at the surgical site must have free and quick access e.g. for security reasons. This somehow contradicts the requirements of a smooth and steady environment required by the strain gage device itself. Thus, it is difficult to ensure a robust and reliable determination of the liquid level or volume of salvaged blood in the blood collection reservoir.
EP 2531823 A1 of the applicant discloses a method and apparatus for the capacitive level measurement of a liquid medium in a bubble trap of a dialysis machine.
U.S. Pat. No. 4,275,726 of the applicant discloses an apparatus and method for balancing the fluids withdrawn from a patient and the fluids simultaneously returned into the patient for use in exchange blood transfusion, infusion therapy in conjunction with forced diuresis or haemofiltration. In this approach the liquid level of a liquid reservoir used for temporarily storing a liquid is sensed by means of a row of radiation emitters and a row of radiation receivers disposed on a side of the liquid reservoir opposite to the radiation emitters, which generate an output signal representative of the transmission at the various different height levels of the collection reservoir. Use of this method for autotransfusion devices is not disclosed. Furthermore, problems caused by obstruction of the light beams transmitted through the reservoir caused e.g. by a blood or lipid film on the inner surface of the reservoir are not handled.
Thus, there exists a need for more robust and reliable concepts for determining the liquid level of salvaged blood in a blood collection reservoir of autologous blood transfusion systems (autotransfusion devices).